In recent years, calls have intensified for cutbacks on chemical fuel consumption in order to protect the environment and prevent global warming, and these demands have had various effects on the manufacturing industry. For example, even the automobile, an indispensable means of transport in daily life and activities, is no exception, and improved fuel efficiency and the like through body weight reduction and other means is being required. In the case of automobiles, however, mere realization of body weight reduction is not a viable option from the viewpoint of product quality, and appropriate safety must also be ensured.
The structure of an automobile is formed largely of steel, particularly steel sheet, and reducing the weight of the steel sheet is essential for vehicle body weight reduction. As just pointed out, however, mere reduction of steel sheet weight is not acceptable because the mechanical strength of the steel sheet must be ensured. Such requirements for steel sheet are not limited to the automaking industry but also apply similarly to various other manufacturing sectors. R&D has therefore been conducted with regard to steel sheet that, by enhancing the mechanical strength of the steel sheet, is capable of maintaining or increasing mechanical strength even when made thinner than the steel sheet used heretofore.
A steel material having high mechanical strength generally tends to decline in shape fixability during bending and other forming, so that the metalworking itself becomes difficult in the case of formation into a complicated shape. One means available for overcoming this formability problem is the so-called “hot stamping method (hot-pressing, high-temperature stamping, die-quenching)”. In the hot stamping method, the steel material to be formed is once heated to a high temperature, whereafter the steel sheet softened by the heating is stamped and then cooled. Since the hot stamping method softens the steel material by once heating it to a high temperature, the material can be readily stamped, while, in addition, the mechanical strength of the material can be increased by the quenching effect of the cooling after the forming. The hot stamping method therefore makes it possible to obtain a formed article that simultaneously achieves good shape fixability and high mechanical strength.
However, when the hot stamping method is applied to a steel sheet, the heating to a high temperature of, for example, 800° C. or higher oxidizes iron and the like at the surface, thereby producing scale (oxide). A process for removing the scale (descaling) is therefore required after conducting the hot stamping, which lowers productivity. Moreover, in the case of a component or the like requiring corrosion resistance, it is necessary to corrosion-proof or metalclad the component surface after fabrication, which makes a surface cleansing step and a surface processing step necessary and also lowers productivity.
As an example of a method for minimizing such loss of productivity can be mentioned that of providing a coating on the steel sheet. Any of various materials, including organic materials and inorganic materials, are generally used for the coating on the steel sheet. Among them, steel sheet having a zinc-based coating that provides the steel sheet with a sacrificial corrosion protection effect is widely used for automotive steel sheet and the like, from the viewpoints of anticorrosion performance and steel sheet production technology. However, the heating temperature in hot stamping (700 to 1000° C.) is higher than, for example, the decomposition temperatures of organic materials and the boiling points of Zn-based and other metallic materials, so that the heating during hot stamping may sometimes evaporate the surface coating layer to cause marked degradation of the surface properties.
Therefore, as a steel sheet to be subjected to hot stamping involving high-temperature heating, it is preferable to use a steel sheet having an Al-based metal coating, which has a higher boiling point than an organic material coating or a Zn-based metal coating, that is, to use a so-called aluminum-plated steel sheet.
Provision of an Al-based metal coating prevents scale from adhering to the steel sheet surface and improves productivity by making a descaling or other such process unnecessary. Moreover, corrosion resistance after painting improves because the Al-based metal coating has a corrosion-proofing effect. Patent document 1 describes a method which performs hot stamping using an aluminum-plated steel sheet obtained by coating a steel having a predetermined steel composition with an Al-based metal coating.
However, when an Al-based metal coating is applied, and depending on the preheating conditions prior to stamping in the hot stamping process, it may happen that the Al coating first melts and is then changed to an Al—Fe alloy layer by Fe diffusion from the steel sheet, whereby Al—Fe compound comes to extend to the steel sheet surface with growth of the Al—Fe composite. This compound layer is hereafter called the alloy layer. As this alloy layer is extremely hard, processing scratches are formed by contact with the die during stamping.
The surface of the Al—Fe alloy layer is by nature relatively resistant to slipping and poor in lubricity. In addition, the Al—Fe alloy layer is relatively hard and susceptible to cracking, so that formability is liable to decrease owing to cracking, powdering and the like of the plating layer. Moreover, the quality of the stamped product is degraded by adhesion of Al—Fe to the die owing to, inter alia, sticking to the die of exfoliated Al—Fe alloy layer and of the strongly scored Al—Fe surface. This makes it necessary to remove the Al—Fe alloy powder adhering to the die during repair, which lowers productivity and increases cost.
In addition, the Al—Fe compound is low in reactivity with ordinary phosphate treatment, so that no film (phosphate film) is produced by the chemical conversion treatment, which is an electrocoating pretreatment. Painting adhesion is good even without formation of a chemical conversion treatment film and corrosion resistance after painting is also good so long as the coating weight of the Al plating is made adequate, but increasing the coating weight tends to aggravate the aforementioned die adherence. As was pointed out earlier, the adherence is sometimes due to attachment of exfoliated Al—Fe alloy layer and sometimes due to attachment owing to strong scoring of the Al—Fe surface. Although the latter problem is ameliorated by increasing the lubricity of the surface film, the beneficial effect with respect to the latter is relatively small. Coating weight reduction is the most effective for improvement in the former case. However, corrosion resistance decreases when the coating weight is reduced. The coating weight also has a major effect on local plating non-uniformity caused by the pinch effect, and unevenness of plating thickness is naturally less likely to occur at a lower coating weight. (The pinch effect will be discussed in detail later.)
In contrast, a steel sheet aimed at preventing processing scratches and the like is taught by Patent Document 2 listed below. Patent Document 2 teaches that a steel sheet of predetermined composition is provided with an Al-based metal coating and the Al-based metal coating is further formed thereon with an inorganic compound film containing at least one of Si, Zr, Ti and P, and an organic compound film, or a complex compound film of these. With the steel sheet formed with such a surface film or films, a surface film remains also during the stamping after heating, so that formation of processing scratches during stamping can be prevented. Moreover, the surface film(s) can serve as lubricant during stamping to enable formability improvement. In actuality, however, adequate lubricity cannot be realized, so that another lubricant or alternative means is required.
On the other hand, the heating to a high temperature prior to stamping melts the Al-based metal coating. Therefore, in the case where, for example, a furnace in which blanks stand vertically during the heating is used, the plating thickness becomes uneven because the molten aluminum plating runs under the force of gravity and the like.
Further, if, for example, resistance heating or induction heating is conducted, a higher temperature increase rate than in atmospheric heating or near-infrared ray (NIR) heating can be achieved, whereby productivity can be improved. However, when the steel sheet is heated by resistance heating or induction heating, the molten aluminum distributes unevenly at some portions owing to the pinch effect, so that the plating thickness becomes uneven. Such unevenness of plating thickness is undesirable from the aspect of product quality, degrades formability during the ensuing stamping, decreases productivity, and by extension is liable to lower corrosion resistance.
In other words, the fact that the aluminum plating melts poses a problem similar to that in galvanized steel sheet. Patent Document 3 teaches a method for overcoming surface degradation by evaporation of the surface zinc plating layer in hot stamping of galvanized steel sheet. Specifically, it teaches formation of a zinc oxide (ZnO) layer of high melting point on the surface of the zinc plating layer to serve as a barrier layer for preventing evaporation and runoff of the underlying zinc plating layer. However, the technique taught by Patent Document 3 assumes a zinc plating layer. Although it allows an Al content of up to 0.4%, it teaches that a lower Al concentration is preferable and is a technique not essentially premised on Al. The technological problem here is Zn evaporation and is therefore naturally a problem that cannot arise in the case of an Al plating of high boiling point.